Phenotypic plasticity & life histories

What is phenotypic plasticity?

An important feature of parasite lifestyles is that their environments change constantly. For example, hosts develop immune responses and anaemia during infections and parasites in different infections may find themselves sharing the host with other con-specific strains or species.

Parasite genotypes exhibit phenotypic plasticity, the capacity to facultatively alter phenotypes, traits, and behaviours (such as how much to invest into sexual, transmission, stages) in response to such changes to their in-host environment.

Plasticity enables organisms to maintain fitness under variable environmental conditions, by altering their phenotype through mechanisms such as differential gene expression. Evolutionary biologists and parasitologists have tended to overlook the notion that plasticity can produce adaptive changes in phenotypes, traits, and behaviours during infections. Instead, parasite responses to environmental perturbation are typically assumed to be directed at maintaining homeostasis. We show that variation in parasite behaviours is often – and incorrectly – attributed to the footprint of host regulation rather than parasites making strategic decisions that maximise their fitness.

What is a life history trait?

An organism’s life history refers to events related to survival and reproduction (i.e. fitness) from birth to death. These events may occur at particular times, ages, or under certain environmental or internal conditions. The traits most commonly studied include juvenile development, age at sexual maturity, first reproduction, number/size of offspring, ratio of male to female offspring, level of parental investment, senescence and death.

Why apply these concepts to parasites?

As for all organisms, parasites experience trade-offs between life history traits, including growth, reproduction and survival. Natural selection will shape life history strategies to maximize fitness in a particular environment and when the environment changes, organisms may be selected to alter life history strategies to maintain fitness. Parasites can adapt to environmental changes by plastic (within infection) alterations to life-history strategies as well as by evolutionary (genetic) changes.

Our work has revealed that parasites have considerably more sophisticated life history strategies than previously thought.

Determining whether this variation is beneficial for parasite fitness is important for medicine. For example, plasticity can generate a wide range of phenotypes that allow parasites to evade the effects of drugs and vaccines on very short timescales. Second, the costs and limits of parasite plasticity may maintain genetic variation in natural populations and could offer novel disease intervention targets. Third, when different genotypes respond to environmental change in different ways, this can either increase or decrease the genetic variation available for natural selection to act on, facilitating or constraining evolution.